In optical communication and data transmission systems it is desirable to be able to couple light energy into and out of optical paths which carry signal information. In the prior art light energy has been coupled out of optical paths in such systems by the use of a grating coupler, for instance, in passive or active forms, as well as through the use of a prism coupler, among other techniques. It has also been suggested by S. E. Miller in Volume 48 of the Bell System Technical Journal beginning at page 2059 that active forms of frequency selective coupling between light paths may be possible through the use of integrated optical circuits.
The frequency dependence of coupling light energy between two identical parallel dielectric waveguides has been theoretically predicted in a number of published scientific articles such as that of A. L. Jones which appeared in Volume 55, of the Journal of the Optical Society of America, beginning at page 261. Such coupling of light energy has also been reported by N. S. Kapany et al in Volume 58 in the Journal of the Optical Society of America beginning at page 1176 as having been experimentally observed in nearly identical glass fibers.
There exists, however, a need for a passive optical coupler which is frequency-selective in its operation for transferring light energy only at known and predeterminable frequencies, in which the frequency selectivity is much more pronounced than that known in the prior art. In many optical systems it is desirable that different signal information be carried at different optical frequencies so that a single optical transmission line may be employed to multiplex a plurality of signals, each conveying its own signal information at a different pre-selected and predetermined frequency. There is therefore a requirement for an optical coupler preferably of the passive, non-active type which can be employed to selectively couple known and predtermined frequencies out of such a multiplex communications system optical path at different points as desired and at different discrete predetermined frequencies for effectively separating the signal information transmitted and conveyed by each such frequency in the multiplexed optical path.